1. Field of the Invention
The invention relates to a method for operating a Coriolis mass flow meter and to a Coriolis mass flow meter.
2. Description of the Related Art
In process automation, it is often required to measure the flow rate of a medium through a pipeline. Devices for this purpose are known, such as magnetic-inductive, ultrasonic or float-type flow meters, and similarly vortex flow meters or differential pressure measuring transducers, which work together with a pressure orifice plate in the measuring tube. These meters directly supply a measured value for the flow rate or, taking into consideration the geometry of the measuring tube, for the flow volume. Only Coriolis mass flow meters allow a direct measurement of the mass flow.
In general, Coriolis mass flow meters have a single measuring tube or a number of measuring tubes, such as a pair, through which a medium flows, and of which the mass flow is to be determined. Various arrangements and geometries of the measuring tubes are known for achieving this. For example, Coriolis mass flow meters with a single straight measuring tube and Coriolis mass flow meters with two curved measuring tubes running parallel to one another are conventional devices. The latter measuring tubes, formed identically as a pair, are induced by an excitation system placed in the middle region to vibrate such that they oscillate in opposition to one another, i.e., the vibrations of the two measuring tubes are phase-offset with respect to one another by 180°, to achieve a mass equalization. The position of the center of mass of the system formed by the two measuring tubes thereby remains substantially constant and forces occurring are largely compensated. As a positive consequence, this has the result that the vibrating system has scarcely any external effect as such. Provided upstream and downstream of the excitation system are vibration pickups, between the output signals of which a phase difference can be evaluated as a measuring signal when there is a flow. This is caused by the Coriolis forces prevailing when there is a flow, and consequently by the mass flow. The density of the medium influences the resonant frequency of the vibrating system. Consequently, apart from the mass flow, it is also possible to determine, inter alia, the density of the flowing medium.
WO 2009/089839 A1 discloses a Coriolis mass flow meter with which the undesired state of a multiphase flow can be detected more clearly, and consequently more reliable operation of the meter can be made possible. Information on multiphase flows, such as two-phase flows, in particular the detection of the occurrence of such a multiphase flow as well as findings concerning the manifestation of the multiphase flows, can be obtained in this way. An example of a two-phase flow is that of gas bubbles in a liquid, which may be caused, for example, by cavitation in valves or pumps or the intake of air at leaks of a pipeline system. Furthermore, an example of a two-phase flow is a mixture of solids in a liquid, for example, caused by crystallization or the sudden detachment of deposits in the pipeline system through which the medium flows. A further example is that of mixtures of insoluble liquids, i.e., emulsions, which may be caused, for example, by a change of the medium flowing through the pipeline system.
DE 10 2006 017 676 B3 discloses a method for operating a Coriolis mass flow meter in which a number of indicator variables are determined for the detection of a multiphase flow. For example, for the determination of an indicator variable that is based on frictional losses within the two-phase flow, the drive power required for inducing the measuring tube to vibrate is divided by the root mean square value of a vibration signal. For determination of another indicator variable, a frequency spectrum of the signals sensed by the vibration pickups is analyzed. Deviations of a spectrum determined during the operation of the Coriolis mass flow meter from a previously established, nominal spectrum are used for the calculation of the further indicator variable for the detection of a multiphase flow.
Many process automation applications involve transporting media, which over time form deposits or adhering attachments on the inner sides of the pipelines and similarly inside the flow meter. With increasing deposits, the measuring accuracy of the Coriolis mass flow meter deteriorates. If the deposits even lead to a partial clogging of the measuring tube, the pressure in the pipelines and in the meter that is required for maintaining the desired flow rate must be increased. This effect is particularly critical in the food and drinks industrial sector, because an excessively increased pressure can adversely influence the quality of the products produced.